Some internal combustion engines may be operated in a four stroke mode, whereby the engine is operated to perform a combustion cycle once every four piston strokes. Other internal combustion engines may be operated in a two stroke mode, whereby the engine is operated to perform a combustion cycle once every two piston strokes. Engines may produce greater maximum torque when operated in the two stroke mode as compared to the four stroke mode since the combustion frequency and therefore the power density of the engine may be greater in the two stroke mode than the four stroke mode. However, engines may demonstrate greater fuel efficiency when operated in the four stroke mode as compared to the two stroke mode, due in part to better air and fuel mixing before combustion of the charge is initiated.
Improvements in valve actuation systems have enabled engines to be operated in the two stroke mode under some conditions and the four stroke mode in other conditions. This approach has enabled improvements in both fuel efficiency and increased maximum torque of the engine. However, the inventors herein have identified some issues with the this approach. As one example, transmission shifting may be increased as a result of the multi-mode engine capability. Increased transmission shifting may result in reduce fuel efficiency, increased transmission wear, and reduced drivability of the vehicle as the transmission shifts may be perceived by the vehicle operator.
The inventors have recognized that transmission shifting may be reduced in some examples by a method of operating a propulsion system for a vehicle, where the propulsion system includes an internal combustion engine coupled to one or more drive wheels of the vehicle via a transmission. As a non-limiting example, the method includes: operating the engine to produce an engine output; transferring the engine output to one or more drive wheels of the vehicle via the transmission; and adjusting a number of strokes performed by the engine per combustion cycle while shifting the transmission between different gear ratios. For example, the engine may be transitioned from the four stroke mode to the two stroke mode while the transmission is up-shifted by reducing the gear ratio of the transmission. In this way, the transmission may be shifted by a greater extent (e.g. to a higher gear ratio) than would otherwise be suitable when the engine is operated in the four stroke mode, while the new gear ratio may be suitable if the engine is transitioned to the two stroke mode providing increased maximum torque potential.
The inventors herein have further recognized that the engine lug limit (i.e. the lower speed limit of the engine where NVH becomes unacceptable) may differ depending on whether the engine is operating in the four stroke mode or the two stroke mode. For example, the inventors have recognized that the lug limit may be lower in the two stroke mode than the four stroke mode as a result of the increased combustion frequency of two stroke operation. As such, a method of operating a propulsion system for a vehicle is described. When the engine is rotating at a lower engine rotational speed, the vehicle speed may be reduced by increasing a number of strokes performed by the engine per combustion cycle while increasing a gear ratio of the transmission; and when the engine is rotating at a higher engine rotational speed, the vehicle speed may be reduced by reducing a number of strokes performed by the engine per combustion cycle while maintaining the transmission at a selected gear ratio.
In this way, transmission shifting during deceleration of the vehicle may be delayed or eliminated, under some conditions, by transitioning the engine from the four stroke mode to the two stroke mode as the engine rotational speed approaches the lug limit of the engine operating in the four stroke mode rather than performing a transmission shift. The use of coordinated mode transitions may enable the transmission may remain in the selected gear ratio for a longer period of time during deceleration of the vehicle, thereby enabling a subsequent up-shift of the transmission to be eliminated when the vehicle operator requests acceleration of the vehicle.